KABUL, Afghanistan — Afghanistan on Thursday announced presidential elections for Aug. 20, hoping a U.S. troop surge will improve security at a time when violence is at the highest levels since the overthrow of the Taliban.

The election had been scheduled for the spring, according to the Afghan constitution, but Azizullah Loden, the head of the country's Independent Election Commission, said the security situation is not strong enough to hold elections then.

U.S.-backed President Hamid Karzai has strongly hinted he will run for re-election, but his popularity has waned due to widespread official corruption and widening violence.

President Barack Obama has ordered a comprehensive review of Washington's strategy in Afghanistan, which a White House official said would have a "significant non-military component," a likely reference to development aid.

The Obama administration is also considering almost doubling the U.S. force in Afghanistan from 36,000 to more than 60,000 to secure the polls. A similar troop surge in Iraq improved security there considerably.

Ensuring Afghanistan holds successful elections is a key marker of progress in the face of a Taliban insurgency that has grown stronger in the ethnic Pashtun heartlands of the south and east and is now encroaching into areas just outside the capital.

U.S. Defense Secretary Robert Gates on Tuesday described Afghanistan as America's greatest military challenge.

The Pentagon's ability to boost its forces in Afghanistan depends partly on how quickly it can withdraw them from Iraq, where 140,000 U.S. troops are deployed.

Forces 'will guarantee security'
Independent Election Commission chief Loden said Aug. 20 was chosen for the presidential poll after consulting with Afghan and international security forces.

"They told us there will be new security forces here ... and they will guarantee security," he told a news conference.

Karzai, an ethnic Pashtun from the south, the country's most violent region, has led Afghanistan since U.S.-led and Afghan forces toppled the Taliban in the wake of the Sept. 11, 2001, attacks, first as head of an interim administration and then after winning elections in 2004.

But since then, the overwhelmingly Pashtun Taliban has regrouped and relaunched an insurgency to overthrow the government and drive out foreign troops, with guerrilla attacks and hundreds of suicide bomb attacks that have killed scores of troops and civilians alike.

Once the darling of the west with his exotic ethnic robe and hat, Karzai has since fallen out of favor with his Western backers due to his failure to rein in rampant official corruption and govern effectively outside the capital, Kabul.

Before his election, Obama said in July that "Karzai has not gotten out of the bunker and helped to organize Afghanistan, and the government, the judiciary, police forces, in ways that would give people confidence."

For his part, Karzai has hit back, strongly criticizing U.S. and NATO forces for killing dozens of civilians in air strikes -- incidents that provoke extreme anger among Afghans and seething resentment against the presence of foreign troops.

While no candidates have officially announced they will run, diplomats say there are few, if any, alternatives to Karzai who would gain support from majority Pashtuns, the traditional rulers of Afghanistan, and be able to govern effectively.

According to the constitution, the elections should be held by May, but officials announced last year that would not be possible as holding polls in the spring would mean organizing them during the harsh winter when large parts of the country are inaccessible.